Solid materials that are self-lubricating when dry are commonly used to fabricate mechanical parts such as bushings, ball joints, or pivots, that are subjected to high levels of friction even though their operating conditions make it impossible to use liquid lubricants of the oil or grease type. This applies in particular to the bushings used for protecting the roots of variable-pitch vanes in the compressors of airplane turbojets.
These bushings are generally mounted as tight fits in orifices formed through the stator casing of the compressor. They receive the roots of the variable-pitch vanes of the compressor. An example of this type of bushing is described in the US patent published under the U.S. Pat. No. 6,480,960 B2.
Such bushing and blade-root assemblies are subjected to large amounts of friction associated with the blades pivoting inside the bushings, or with the vibration caused by the operation of the turbojet. The bushings are made of a material that is “softer” than the material used for the pivots so that it is the bushings that wear as a priority, thereby protecting the pivots.
In order to limit the wear of said bushings (and thus the frequency with which they need to be replaced), it is advantageous to reduce friction at the contacting surfaces between the bushings and the vane pivots. That is why such bushings are made of a self-lubricating solid material, by sintering an intimate mixture of powders.
Such a mixture generally comprises a powder of a metal alloy, acting as a precursor for the matrix of the self-lubricating material, together with particles of a solid lubricant that are stable at the temperatures at which the material is worked and used so that they do not react with said metal alloy and remain intact in order to be capable of performing their lubricating action. Naturally, the greater the content of such particles in the mixture, the better are the self-lubricating properties of the final material (where the term “final material” is used to mean the material made from said powder mixture).
Nevertheless, the Applicant company has found that beyond a certain content level of this type of solid lubricant in the intimate mixture, evaluated as being 10% by volume, problems of densification appear, and the powder mixture becomes more difficult to sinter. In practice, it becomes necessary to increase the temperature and the duration of sintering or to use more complex pressing techniques, such as hot isostatic pressing, in order to be able to densify the powder mixture, thereby leading to an increase in the cost price of the fabricated parts. In any event, the final material presents a high degree of porosity and its mechanical properties suffer accordingly.
Furthermore, beyond a limiting content of lubricant, evaluated at 15% by volume, it is generally found that it becomes very difficult, if not impossible, to sinter the powder mixture.